A laser diode is a semiconductor diode, the p-n junction of which functions as a laser gain medium. To obtain population inversion in the gain medium, a forward current is applied to the p-n junction. Cleaved or polished facets of the laser diode chip may be used to form a laser cavity. An external mirror and/or a Bragg reflector may also be used for this purpose.
Laser diodes are inexpensive and efficient sources of coherent light at high power density and brightness. Laser diodes are widely used in electro-optical devices ranging from CD players to concrete-cutting industrial lasers. In industrial lasers, laser diodes are frequently employed as a pump source for rare earth doped optical fibers or rods. Laser diodes are also widespread in optical fiber amplifiers, where they are used to pump erbium doped optical fibers.
Concentrating laser diode light enables brighter, more powerful light sources. By way of example, one may increase an effectiveness of laser diode fiber coupling by using beam forming and directing optics to stack laser beams generated by individual laser chips on top of one another to better fill the optical fiber's circular input aperture. Alternatively or in addition, individual laser beams may be overlapped to co-propagate in space; however for the laser beams to be efficiently overlapped, they must differ in some property, such as wavelength or polarization. When the beams are overlapped, the brightness of the resulting beam is increased. Increasing brightness of laser diode light sources is important for many applications, including pumping of fiber lasers and processing of materials directly with diode radiation.
Emission of laser diodes at different wavelengths may be combined using dichroic mirrors. Referring to FIG. 1, a prior-art laser diode light source 100 includes laser diode emitters 120, external volume Bragg reflectors (VBG) 101, 102, and 103 for providing an optical feedback to the laser diode emitters 120 at different respective wavelengths λ1, λ2, and λ3 within a spectral gain band of the laser diode emitters 120 to generate first 111, second 112, and third 113 laser beams at the respective wavelengths λ1, λ2, and λ3. First 121 and second 122 dichroic mirrors combine the laser beams 111, 112, and 113 to obtain an output laser beam 119. In the example shown, the first dichroic mirror 121 transmits the first laser beam 111 at the first wavelength λ1, while reflecting the third laser beam 113 at the third wavelength λ3. The second dichroic mirror 122 transmits the first laser beam 111 at the first wavelength λ1 and the third wavelength λ3, but reflects the second laser beam 112 at the second wavelength λ2.
One drawback of the laser diode light source 100 is bulkiness. For the first 121 and second 122 dichroic mirrors to reflect light in wavelength-selective manner in both polarizations of light, angles of incidence of the respective third 113 and second 112 laser beams on the first 121 and second 122 dichroic mirrors need to be small, e.g. 12-25 degrees, precluding close placement of the first 121 and second 122 dichroic mirrors. Another drawback is related to having the individual VBG 101 to 103 for each laser diode emitter 120, increasing the cost of the laser diode light source 100. Both drawbacks may become more prevalent as one scales the power up by increasing the number of individual laser diode emitters 120 in the laser diode light source 100.